On the heat capacity of liquids at high temperatures

Making use of a simple approximation for the evolution of the radial distribution function, we calculate the temperature dependence of the heat capacity Cv of Ar at constant density. Cv decreases with temperature roughly according to the law ∼T−1∕4, slowly approaching the hard sphere asymptotic value Cv=32R. However, the asymptotic value of Cv is not reachable at reasonable temperatures, but stays close to 1.6-1.7 R over a wide range of temperatures after passing a “magic” 2R value at about 300 K. Nevertheless these values have nothing to do with loss of vibrational degrees of freedom, but arise as a result of a temperature variation of the collision diameter σ.